Bonding member for semiconductor device

ABSTRACT

A bonding member  10  used for bonding a semiconductor device  20  and a substrate  30 , the bonding member including: a thermal stress relieving layer  11  made of any of Ag, Cu, Au, and Al; a first Ag brazing material layer  12  containing Ag and Sn as main components and provided on a side of the thermal stress relieving layer to which the semiconductor device is bonded; a second Ag brazing material layer  13  containing Ag and Sn as main components and provided on a side of the thermal stress relieving layer to which the substrate is bonded; a first barrier layer  14  made of Ni and/or Ni alloy and provided between the thermal stress relieving layer and the first Ag brazing material layer; and a second barrier layer  15  made of Ni and/or Ni alloy and provided between the stress relieving layer and the second Ag brazing material layer, in which a thermal conductivity of the bonding member after a power cycle test is 200 W/m·K or more.

TECHNICAL FIELD

The present invention relates to a bonding member used for bonding asemiconductor device and a substrate such as one used as both electrodeand heat release substrate (which will be referred to as an “electrodeand heat release substrate”).

BACKGROUND ART

Electrically powered vehicles (for example, an electric vehicle (EV))are becoming widespread in countries around the world. Under suchcircumstances, efforts to downsizing and high performance of an inverteror the like using a Si semiconductor device or a SiC semiconductordevice have been made for use in a motor control device or the like.

In a small and high-performance semiconductor module such as aninsulated gate bipolar transistor (IGBT), it is necessary to release alarge amount of heat generated in a semiconductor device to an electrodeand heat release substrate in order to cool the semiconductor device.The linear expansion coefficient of Si, SiC, or GaN constituting thesemiconductor device (in many cases, a Ni layer or the like serving asboth an electrode and a bonding layer is provided on a bonding surfaceof the semiconductor device.) is as small as in a range of 4 to 6 ppm/K.On the other hand, the linear expansion coefficient of Cu constitutingthe electrode and heat release substrate is as large as 17 ppm/K.Considering the heat release efficiency from the semiconductor device,it is desirable to bond the electrode and heat release substratedirectly, i.e. without interposing anything, to the semiconductordevice, which is a heat generating element. A technique of a high vacuumpressure heating method has been developed in order to perform suchbonding. However, materials of the semiconductor device and the heatrelease substrate are different from each other, and as described above,there is a large difference in linear expansion coefficient between thesemiconductor device and the heat release substrate. It is well knownthat when such a semiconductor device as IGBT works, heat is generatedfrom the semiconductor device, and thermal stress occurs at a bondingface between the semiconductor device and the electrode and heat releasesubstrate, resulting in a separation between the semiconductor deviceand the electrode and heat release substrate. No device in which thesemiconductor device and the electrode and heat release substrate aredirectly bonded has yet been put into practical use. At present, thethermal stress is alleviated by bonding the semiconductor device and theelectrode and heat release substrate with flexible solder of a certainthickness. Since the stress relaxing ability is insufficient when thethickness is 10 μm or less, many products are made in which the twomembers are bonded by solder having a thickness of 30 to 300 μm (often50 to 150 μm). In particular, in such a semiconductor device as IGBTwhich generates a large amount of heat, a die bond (bonding) techniquefor bonding a semiconductor device and an electrode and heat releasesubstrate is important.

Since an electrode and heat release substrate to be attached to asemiconductor device of an IGBT or the like is required to haveelectrical conductivity, a substrate made of Cu or Cu alloy is mainlyused. However, in a case where a semiconductor device is bonded to anelectrode and heat release substrate made of a thick and large Cu plateby Pb-based or Sn-based solder, there is a problem that when such asemiconductor device as IGBT works, heat is generated in thesemiconductor device, and the semiconductor device and the electrode andheat release substrate separate due to a difference in linear expansioncoefficient. In order to cope with this problem, in mass-producedproducts, a direct bonded cupper (DBC), in which the apparent linearexpansion coefficient of the electrode and heat release substrate isreduced to 7 to 10 ppm/K by bonding thin Cu foils to upper and lowersurfaces of a ceramic substrate by various methods (including Ag brazingusing AgCuSnTi or the like having high heat resistance and high thermalconductivity), and a semiconductor device are bonded by Pb-based orSn-based solder so that the highest operation temperature is raised to150° C. Furthermore, an IGBT in which the highest operation temperatureis raised to 175° C. by devising a type of solder has also beenreported. However, it is considered that there is a limit to furtherincrease the operation temperature. In addition, if the Cu foil is madethinner than the thickness of the ceramic, the linear expansioncoefficient of the DBC can be decreased, but there is another problemthat the thermal conductivity decreases. In recent years, DBC in whichthick Cu is attached has been developed in order to improve thermalconductivity, but when Cu is thickened, the value of linear expansioncoefficient of the DBC becomes close to that of Cu, and separation mayoccur due to thermal stress in bonding by solder. There is also a methodin which an electrode part and an insulating part are separatelyprepared, but it is difficult to bond a thick and large Cu plate and theceramic substrate with solder. Furthermore, for bonding the ceramic anda cooler, thermal grease is used which is a flexible resin. However,since the resin has a very low thermal conductivity of 0.1 to 1 W/m·K,there is a restriction that heat needs to be released by a method ofcooling both surfaces of the semiconductor device.

In recent years, developments have been actively conducted focusing onsoldering, but even when Pb-based or Sn-based solder having the meltingpoint as high as 200° C. or higher is used, a problem occurs in a heatcycle test at 200° C. Even in the case of Au solder having the meltingpoint of 356° C., the same problem occurs in the heat cycle test at 225°C. It has been found that these are caused by the fact that amicro-structure of the solder greatly changes and the solderdeteriorates when a heat cycle test at 200° C. or higher is performed.

Therefore, developments of new bonding materials in place of solder havebeen made. A representative one of such bonding members uses Agnano-particles (nano-sized Ag particles). When Ag nano-particles areused, it is possible to bond the semiconductor device and the electrodeand heat release substrate to each other at a low temperature of about200° C., and it is expected to obtain characteristics that thermalconductivity is as high as 200 W/m·K or more and the melting point afterbonding is also as high as 960° C. For example, Non Patent Literatures 1to 3 describe a phenomenon in which Ag nano-particles can be sintered(low-temperature sintering) at a temperature lower than the meltingpoint of the bulk material due to their high surface activity. However,Patent Literature 2 reports that expected properties are significantlydeteriorated in a heat test in which the bonding member formed bysintering Ag nano-particles is left at 250° C. Non Patent Literature 3reports, regarding a power semiconductor module using Ag nano-particlesas a die bond material, that, from a result of a heat cycle test inwhich cooling to −40° C. and heating to 125° C. are repeated, expectedreliability similar to that in the case of using solder as a die bondmaterial can be obtained. However, in this bonding member, there areparts where secondary particles grown from the Ag nano-particles are notsintered (unsintered parts of the secondary particles) and voids.Therefore, sufficient strength cannot be obtained, and when a heat cycletest reaching 300° C. is performed, cracking or chipping starting fromgrain boundaries likely occurs, so that it is difficult to bond asemiconductor device having a highest operation temperature of 300° C.

The sintering start temperature of a general Ag powder is 450° C. orhigher. When the Ag powder is sintered under a load of 30 MPa andtemperature of 900° C., a sintered body having a thermal conductivity(420 W/m·K) equivalent to that of a pure Ag plate is obtained. Whensintering is performed at a pressure lower than this, the amount ofvoids existing in the sintered body increases. There is a correlationbetween the amount of voids included in the body and the thermalconductivity of the body. Since the Ag nano-particles are very fine, itis said that the Ag nano-particles are uniformly sintered, but strictlyspeaking, it is also said that a large number of microvoids are present.Furthermore, it is said that the thermal conductivity of the sinteredbody with the bonding temperature set to 300° C. was 250 W/m·K, which isconsidered to be a result of partial progress of sintering of thesecondary particles. On the other hand, it is also said that large voidsare generated in this sintered body. For example, when the thermalconductivity of the prepared sintered body is 200 W/m·K, there is a 52%decrease in thermal conductivity, as 1−(200/420)=0.52, which reflectsthat unsintered parts of the secondary particles and microvoids arepresent in 52% of the sintered body due to insufficient sintering. Whenthe thermal conductivity is 250 W/m·K, the thermal conductivitydecreases by 40%, as 1−(250/420)=0.40, which reflects that voids largerthan microvoids are present in addition to that unsintered parts of thesecondary particles and microvoids are present in 40% of the sinteredbody due to insufficient sintering. This is considered to be a cause offatigue deterioration of the nano-Ag bonding member in the heat test orthe heat cycle test. In addition, the reason why the use of the Agnano-particles has not progressed even in the IGBT having a lowoperation temperature is that the price of the Ag nano-particles is veryexpensive and higher than that of bulk Au (50 to 100 times higher thanthe bulk Ag nominal value), and there is no possibility that the pricewill significantly decrease in the future.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 6834979 B2-   Patent Literature 2: JP 2013-229474 A-   Patent Literature 3: JP 2019-36603 A-   Patent Literature 4: JP 2019-79960 A

Non Patent Literature

-   Non Patent Literature 1: Katsuaki Suganuma et al., “System    Integration of Wide Band Gap Semiconductors”, May 31, 2016, CMC    Publishing Co., Ltd.-   Non Patent Literature 2: Daisuke Hiratsuka et al., “DIE-BONDING    MATERIAL AND SINTERING JOINING TECHNOLOGY FOR POWER SEMICONDUCTORS    ALLOWING OPERATION AT HIGH TEMPERATURES”, TOSHIBA CORPORATION, Vol.    70, No. 11, 2015, p. 46-49, November 2015-   Non Patent Literature 3: Toshiaki Morita, “HIGH THERMAL RESISTANCE    AND LOW THERMAL RESISTANCE Pb-FREE JUNCTION TECHNIQUE USING Ag    NANOPARTICLES AND DEVELOPMENT TO POWER SEMICONDUCTOR MODULE    MOUNTING”, December 2008, Osaka University, Graduate School of    Engineering-   Non Patent Literature 4: Shinichi Yasaka et al., “STUDY OF POWER    CYCLING TEST METHODS FOR EVALUATION OF DIE ATTACH MATERIALS”, Oct.    1, 2016, Kanagawa Industrial Technology Center-   Non Patent Literature 5: Akira Morozumi et al., “Reliability design    technique in power semiconductor module”, Feb. 10, 2001, Fuji    Electric Co., Ltd.-   Non Patent Literature 6: Koji Yamaguchi et al., “Quality and    Reliability Integration Technology in Automotive Semiconductor    Products”, Mar. 10, 2011, Fuji Electric Co., Ltd.

SUMMARY OF INVENTION Technical Problem

The present inventors have proposed a new bonding member in PatentLiterature 1. Under the condition that the semiconductor device will notbreak, the semiconductor device and the electrode and heat releasesubstrate are bonded by a liquid phase diffusion method (including aliquid phase sintering method, a reaction sintering method, a transitionsintering method, and an infiltration sintering method.) using Sn and atleast one of Ag, Cu and Au. In this bonding member, voids generated atthe time of liquid phase diffusion are used for alleviating thermalstress generated between the semiconductor device and the electrode andheat release substrate. In this bonding member, by using an alloycontaining Ag or the like and Sn as main components, a decrease inthermal conductivity due to the presence of voids and breakage occurringfrom the voids as a starting point are avoided.

Specifically, the bonding member is made of an alloy containing Sn andat least one of Ag, Cu and Au as main components and having the meltingpoint of 500° C. or higher, and has a plurality of voids inside, whosetotal volume is 5% or more and 40% or less of the entire volume. Thisbonding member has been developed mainly for the purpose of bonding alarge semiconductor device and an electrode and heat release substrate.In Patent Literature 1, for example, a semiconductor device and anelectrode and heat release substrate are bonded by a liquid phasediffusion method in which powder of Ag and Sn is contained as a maincomponent and a pressure of 1 MPa is applied for 5 minutes while beingheated to 300° C. in a non-oxidizing atmosphere, thereby Sn is meltedand diffused into an Ag plate material (including voids). Furthermore,an SiC semiconductor device plated with Sn and a Cu electrode and heatrelease substrate plated with Sn are disposed on each of both surfacesof an Ag plate material to prepare a laminate, and Sn is melted anddiffused into the Ag plate material (including voids) by applying apressure of 1 MPa for 5 minutes while being heated to 300° C. in anon-oxidizing atmosphere, thereby bonding the semiconductor device andthe electrode and heat release substrate.

The bonding member described in Patent Literature 1 has excellentcharacteristics that the thermal conductivity is 120 W/m·K or more andthe electrical conductivity is 50% IACS or more after a heat cycle testin which cooling to −40° C. and heating to 300° C. are repeated 300times. This is considered to be because thermal stress was relaxed byvoids existing inside the bonding member.

Conventionally, in order to evaluate characteristics of a bondingmember, a heat test in which a test piece obtained by bonding asemiconductor device and an electrode and heat release substrate bymeans of a bonding member is put in an oven and heated, and a heat cycletest in which heating and cooling are repeated every 30 minutes areperformed. However, the temperature rise during the operation of thesemiconductor device in the actual IGBT occurs in several seconds aftersupply of electric current, and the temperature of the bonding partrises more rapidly than heating by an oven. Therefore, a method ofreproducing a temperature rise by current supply between thesemiconductor device and the electrode and heat release substrate andperforming evaluation by a power cycle test in which heating in secondsand cooling in tens of seconds are repeated is close to an actualoperation. For this reason, in particular, in a case where asemiconductor module having a high operation temperature is assumed, itis regarded as a problem that even if the semiconductor module passes aheat keep test or a heat cycle test in which the semiconductor module isheated in an oven, the semiconductor module fails a power cycle test. Asdescribed above, the bonding member described in Patent Literature 1 hasexcellent characteristics that the thermal conductivity is 120 W/m·K ormore and the electrical conductivity is 50% IACS or more after the heatcycle test. However, when a power cycle test in which heating to 300° C.and cooling to 25° C. are repeated is performed this time, the bondingmember deteriorates due to fatigue, and the thermal conductivity and theelectrical conductivity decrease. Moreover, it was found that crackingor breakage of the bonding part occurred.

An object of the present invention is to provide a bonding member thatcan be used for bonding a semiconductor device in which a highestoperation temperature of a semiconductor module reaches 300° C., and anelectrode and heat release substrate, and has sufficient resistance tocurrent supply between the semiconductor device and the electrode andheat release substrate.

Solution to Problem

Conventionally, the melting point of a bonding member has been used asan index of heat resistance, but the present inventors have found thatin Pb-based, Sn-based, or Au-based solder, although the melting point is200° C. or higher, when a heat test is performed at a temperature of200° C. or higher and lower than the melting point, the micro-structureof the material changes and deteriorates. The present inventors havealso found that, though bonding strength before and after the heat testhas been used as an index, it is merely a reference, and when there is adifference in linear expansion coefficient between members bonded bythese solders, deterioration becomes larger. Furthermore, as describedin Non Patent Literature 2, even in a member obtained by sintering Agnano-particles having a melting point of 960° C. or higher,deterioration derived from unsintered parts of secondary particles andmicrovoids generated by unsintering in a power cycle test at 225° C.occurs. Moreover, as described above, even in the bonding memberdescribed in Patent Literature 1, a problem was revealed in the powercycle test at 300° C. Therefore, these cannot be used as bonding memberscorresponding to semiconductor devices operating at 300° C. Othervarious inventions have been made, but none have been found that can beused to bond semiconductor devices having a highest operationtemperature of 300° C. (for example, Patent Literatures 3 and 4).

Furthermore, in order to efficiently release the heat generated in thesemiconductor device to the electrode and heat release substrate, thethermal conductivity of the bonding member needs to be sufficientlyhigh. In order to incorporate an IGBT in a motor of an electric vehicleor the like, a development target of downsizing an IGBT to about 1/10 ofan initial model is shown. Semiconductor devices have already beendownsized to about ⅕ of conventional ones. The highest operationtemperature of a semiconductor device in a semiconductor module usinghigh-temperature Pb-based solder as a bonding member is said to reach175° C. at highest, but is generally about 150° C.

The thermal conductivity of the catalog value of the Pb-based solder isabout 23 W/m·K (20 W/m·K according to the present inventors'measurement). Assuming that the heat generated in the semiconductordevice is double the conventional heat and the heat is released through⅕ of conventional bonding area (i.e. the contact area between thesemiconductor device and the bonding member), the value of the thermalconductivity required for a new bonding member is at least 20W/m·K×2×5=200 W/m·K. This is almost the same as the thermal conductivityof the bonding member using Ag nano-particles. As described above, inmany of conventional die bonds, the bonding strength was evaluated afterthe heat test or the heat cycle test, but in the present invention, therequirement is set so that the thermal conductivity after the powercycle test is 200 W/m K or more because it is actually required in theIGBT or the like.

As described above, since the difference in linear expansion coefficientbetween the semiconductor device and the electrode and heat releasesubstrate is large, thermal stress is generated between thesemiconductor device and the electrode and heat release substrate. Ifthe factor of the thermal stress generated due to the difference inlinear expansion coefficient is considered alone, it seems preferablethat the linear expansion coefficient of the solder of the bondingmember or the nano-Ag is a value between the semiconductor device andthe electrode and heat release substrate. However, conventionally,thermal stress is relaxed by solder having a linear expansioncoefficient larger than that of Cu, and a semiconductor device and anelectrode and heat release substrate are bonded by solder having athickness of 10 μm or more. Furthermore, in a high-performance IGBT, thesemiconductor device and the electrode and heat release substrate arebonded by solder having a thickness of 30 to 300 μm (often 50 to 150μm). Therefore, the present inventors thought that thermal stress may berelaxed by another characteristic more significant than the linearexpansion coefficient, and have come up with the concept of using astress relieving layer made of Ag which is a metal having flexibility(i.e. the Young's modulus is small) and high thermal conductivity.

The present invention made in order to solve the above problems is abonding member used for bonding a semiconductor device and a substrate,the bonding member including:

-   -   a thermal stress relieving layer made of any of Ag, Cu, Au, and        Al;    -   a first Ag brazing material layer containing Ag and Sn as main        components and provided on a side of the thermal stress        relieving layer to which the semiconductor device is bonded;    -   a second Ag brazing material layer containing Ag and Sn as main        components and provided on a side of the thermal stress        relieving layer to which the substrate is bonded;    -   a first barrier layer made of Ni and/or Ni alloy and provided        between the stress relieving layer and the first Ag brazing        material layer; and    -   a second barrier layer made of Ni and/or Ni alloy and provided        between the stress relieving layer and the second Ag brazing        material layer,    -   in which the thermal conductivity is 200 W/m·K or more after a        power cycle test in which heating to 300° C. by current supply        and cooling to 25° C. are repeated 30,000 times.

The present inventors closely analyzed the defects that occurred in aheat keep test, a heat cycle test, and a power cycle test of aconventional bonding member, and scrutinized a phenomenon that occurredin a bonding part. Then, the modes in which the bonding part was brokenin the power cycle test were classified into breakage occurring at theinterface between the semiconductor device and the bonding member and/orthe interface between the bonding member and the electrode and heatrelease substrate (failure mode 1), and breakage occurring inside thebonding member (failure mode 2). It has been found that most of thefailure mode 1 occurs at the interface between the semiconductor deviceand the bonding member, and rarely occurs at the interface between thebonding member and the electrode and heat release substrate. This isconsidered to be because the linear expansion coefficients of thebonding member and the electrode and heat release substrate arerelatively close to each other, and both of the bonding member and theelectrode and heat release substrate are made of a material softer thanSiC or the like constituting the semiconductor device. In conventionalbonding members such as solder, sintered bodies of Ag nano-particles,and bonding members described in Patent Literature 1, these two failuremodes have been basically solved with a single composition or structure.

The present inventors have investigated a cause of a problem that hasoccurred in the power cycle test performed on the bonding memberdescribed in Patent Literature 1. As a result, when the test piece isheated to 300° C. by current supply from the substrate to thesemiconductor device, a large electric current corresponding to theamount of heat flows through the entire bonding member in a short time.Then, it was found that degradation occurred at locally weak part suchas the interface of the bonding member or inside the bonding member, andbreakage at a part of the bonding interface between the semiconductordevice and the bonding member (failure mode 1) and breakage inside thebonding member (failure mode 2) occurred. It was confirmed that thesebreakage sites were locally weak parts in the interface or inside of thebonding member in which voids were formed and the structure became thin,or intermetallic compounds of Ag and Sn were aggregated.

As described above, the failure mode 1 occurs mostly at the interfacebetween the semiconductor device and the bonding member. This isconsidered to be because the high-temperature heat generated when thesemiconductor device is working is first transmitted as the impact ofthe thermal transient, and the load is applied to the bonding membersince the material constituting the semiconductor device is notflexible. Therefore, it is necessary to note these points. Furthermore,it is necessary to bond the semiconductor device under the conditionthat the semiconductor device will not be destroyed. In a SiCsemiconductor device, breakage of the semiconductor device does notoccur at a temperature of about 450° C. and a pressure of about 30 MPa,but in order to more reliably avoid breakage of the semiconductordevice, it is preferable that the semiconductor device is bonded at atemperature of 430° C. or less and a pressure of 15 MPa or less. It isalso preferable that the bonding time does not exceed 60 minutes. Instudying the present invention, it was considered to use Agnano-particles instead of the Ag brazing material, but it was found thata bonding member using a sintered body of Ag nano-particles did not passthe power cycle test at 225° C. or higher. Based on the results, thepresent inventors have focused on improving the Ag brazing material inPatent Literature 1. The invention described in Patent Literature 1 hasled to a bonding technique that passes the heat cycle test at 300° C.,but this bonding member does not pass the power cycle test at 300° C.

The Ag brazing material is a conventionally known material. For example,in an IGBT module, a Ti-containing Ag brazing material or the like,which is a kind of Ag brazing material, is used when a semiconductordevice is bonded to a direct bonded cupper (DBC), which is an insulatingcircuit board in which Cu having excellent conductivity is provided as acircuit layer on a surface of an insulating ceramic substrate(63Ag35Cu1Sn-balance-Ti melting method; brazing temperature 800° C.).The thermal conductivity of this brazing material is 170 W/m·K, which ishigher than the thermal conductivity of the high-temperature Pb-basedsolder or the Sn-based solder (23 to 49 W/m·K). As another type of Agbrazing material, for example, BAg-18 (melting method 60Ag30Cu10Sn),which is one of Ag brazing materials specified in JIS Z 3261, has highthermal conductivity. Furthermore, a AgSn alloy obtained by removing Cufrom BAg-18 also has high thermal conductivity. These Ag brazingmaterials are considered to have good bondability with metals of Cu, Ni,Ag, and Au. However, the brazing temperature of the Ag brazing materialor the like is as high as 720 to 840° C., and the semiconductor deviceis destroyed by heat. On the other hand, the thermal conductivity of thebonding member obtained by transition-sintering a low Ag content of lessthan 50 wt % was low, and the bonding strength was also low. Inparticular, in the bonding member obtained by transition-sintering theAg powder and the Sn powder, instability of bonding due to voids and adecrease in thermal conductivity were clearly observed.

Solder and Ag brazing material are both a type of brazing material. Ingeneral, a solder is considered to have the melting point or the bondingtemperature of 450° C. or lower, and brazing material is considered tohave the melting point or bonding temperature of 450° C. or higher.Furthermore, brazing material containing 50 wt % or more of Ag is calledAg brazing material. In the present specification, brazing materialhaving Ag content of 50 wt % or more is referred to as Ag brazingmaterial. The Ag brazing material has high heat resistance and highthermal conductivity, but the bonding temperature (or the melting pointof the Ag brazing material) is 450° C. or higher, which is so high thatthe semiconductor device may be destroyed.

As described above, the bonding (brazing) temperature of the Ag brazingmaterial is high, and the Ag brazing material cannot be used for bondingsemiconductor devices by melting. However, as described in PatentLiterature 1, if a liquid phase diffusion method in which Sn is meltedand diffused into Ag is used, an Ag brazing material layer can be formedon the bonding surface of the semiconductor device at a low temperatureof about 300° C. The low-temperature bonding Ag brazing that can bebonded under conditions that do not destroy semiconductor devices is apromising bonding method for solving the failure mode 1.

Advantageous Effects of Invention

In the bonding member according to the present invention, the failuremode 1 is eliminated by improving the Ag brazing material. Furthermore,the failure mode 2 is eliminated by using a thermal stress relievinglayer made of pure Ag or the like. This makes it possible to furtherincrease the thermal conductivity. By combining these, the invention ofthe bonding material that passes the power heat cycle test at 300° C.has been achieved. The total thickness of the first Ag brazing materialand the second Ag brazing material is preferably 10% or less of thetotal thickness of the bonding member.

As described above, the bonding member described in Patent Literature 1can be manufactured from Ag powder and Sn powder by bonding under acondition that the semiconductor device is not broken by the liquidphase diffusion method, and passes the heat cycle test at 300° C.However, a weak part caused by voids provided for relaxing the thermalstress became a starting point, and was degraded and failed in the powercycle test.

When a bonding member is made of a single material, even a flexiblematerial needs to have a certain thickness in order to alleviate thermalstress generated between the semiconductor device and the electrode andheat release substrate which have a large difference in linear expansioncoefficient. On the other hand, in the present invention, the thermalstress is relaxed by the thermal stress relieving layer made of pure Ag.Therefore, it is not necessary to relax the thermal stress by the firstbrazing material layer and the second brazing material layer, and it isonly necessary to bond the semiconductor device and the electrode andheat release substrate, so that the thickness can be reduced.

AgSn brazing material constituting the first brazing material layer andthe second brazing material layer exhibits excellent bondability to Ag,Cu, Au, and Al. Furthermore, as described later, by preparing Ag brazingmaterial from Ag foil, Sn foil or the like by a liquid phase diffusionmethod, voids can be reduced, so that weak parts can be reduced andbonding reliability can be improved. Moreover, since the first brazingmaterial layer and the second brazing material layer can be made thin asdescribed above, the gas easily escapes during preparation even at asmall load, and the internal voids can be suppressed to 5 vol % or less.Then, generation of voids can be suppressed as compared with the case ofusing Ag powder or Sn powder as in Patent Literature 1. Even in abonding member made of a general solder or Ag brazing material, whenvoids are present at an amount of 5 vol % or more, there is a highpossibility that separation or breakage occurs.

By suppressing the generation of voids as described above, the thermalconductivity of the first Ag brazing material layer and the second Agbrazing material layer can be increased (the decrease in thermalconductivity due to voids can be suppressed). For example, 65Ag35Sn (Agbrazing material composed of 65 wt % Ag and 35 wt % Sn) had a thermalconductivity of 110 W/m·K (for reference, the thermal conductivity of Niis 90 W/m·K).

Furthermore, in the bonding member according to the present invention,the failure mode 2 is eliminated by using the thermal stress relievinglayer made of any one of Ag, Cu, Au, and Al, and resistance to the powercycle test at 300° C. is imparted. When the semiconductor device and theelectrode and heat release substrate are bonded to each other by AgSnbrazing material by the liquid phase diffusion method, Sn may bediffused into the thermal stress relieving layer to change thecomposition of the thermal stress relieving layer, or voids may begenerated in the thermal stress relieving layer. In the presentinvention, by providing a barrier layer made of Ni or Ni alloy,diffusion of Sn into the thermal stress relieving layer is suppressed.

Since the bonding member according to the present invention includes thethermal stress relieving layer made of any one of Ag, Cu, Au, and Al,the Ag brazing material layers mainly composed of Ag and Sn, and barrierlayers made of Ni and/or Ni alloy, the bonding member has sufficientresistance to current supply, and has a thermal conductivity of 200W/m·K or more as a whole even after the power cycle test is performed.As described above, the bonding member according to the presentinvention can be used for bonding a semiconductor device, in which thehighest operation temperature reaches 300° C., and an electrode and heatrelease substrate of a semiconductor module, and has sufficienttoughness against failure due to current supply between thesemiconductor device and the electrode and heat release substrate.

In the bonding member according to the present invention, as describedabove, the thermal stress relieving layer made of Ag or other metals isused in order to avoid breakage occurring inside the bonding member(failure mode 2). For the thermal stress relieving layer, flexiblematerial is effective, and rubber or resin meets the requirements.Young's modulus is known as an index representing physicalcharacteristics related to flexibility of a material, and for example,Patent Literatures 3 and 4 refer to Young's modulus of bondingmaterials. Solder is an alloy of Pb and Sn, and has a low Young'smodulus of 17 to 68 GPa, but its thermal conductivity is low, and thestructure changes and deteriorates in the heat test at 200° C. orhigher. On the other hand, in the present invention, the thermal stressrelieving layer is made of Ag or other metals having high thermalconductivity. For example, the Young's modulus of pure Ag is 73 GPa,which is smaller than the Young's modulus of SiC (410 GPa) and theYoung's modulus of Cu (120 GPa). Therefore, the thermal stress generatedbetween the semiconductor device and the electrode and heat releasesubstrate is relieved owing to the deformation of the thermal stressrelieving layer made of Ag or other metals. Furthermore, since thethermal conductivity of Ag is as high as 420 W/m·K, heat generated fromthe semiconductor device can be efficiently released to the substrate.It is also possible to further reduce the Young's modulus by using askeleton in which voids are formed (a member whose framework is made ofpure Ag, etc. other than Ag nano-particles. For example, a plate of pureAg, etc. with voids formed inside).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a state in which asemiconductor device and an electrode and heat release substrate arebonded by a semiconductor device bonding member according to anembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

A semiconductor device bonding member according to the present inventionhas a configuration having a function corresponding to each of thefailure mode 1 and the failure mode 2 occurring at a bonding partbetween a semiconductor device and an electrode and heat releasesubstrate in a semiconductor module. Solder, nano-Ag, and the bondingmember described in Patent Literature 1, which are conventional bondingmembers, all bond the semiconductor device and the electrode and heatrelease substrate with a single member. On the other hand, in thepresent invention, a member corresponding to each failure mode isprepared, and these members are combined to solve the problem.

The requirements of the semiconductor device bonding member according tothe present invention are as described above, but the technical idea canbe applied to a wider range. Specifically, for example, a materialconstituting the barrier layer can be made of Co or Co alloy. That is, asemiconductor device bonding member extending the technical idea of thepresent invention can be expressed as a semiconductor device bondingmember including:

-   -   a thermal stress relieving layer made of one or more of Ag, Cu,        Au, and Al;    -   a first Ag brazing material layer containing Ag and Sn as main        components and provided on a side of the thermal stress        relieving layer to which the semiconductor device is bonded;    -   a second Ag brazing material layer containing Ag and Sn as main        components and provided on a side of the thermal stress        relieving layer to which the substrate is bonded;    -   a first barrier layer made of Ni, Co, and/or alloy of Ni and/or        Co, and provided between the stress relieving layer and the        first Ag brazing material layer; and    -   a second barrier layer made of Ni, Co, and/or alloy of Ni and/or        Co, and provided between the stress relieving layer and the        second Ag brazing material layer,    -   in which a thermal conductivity is 200 W/m·K or more after a        power cycle in which heating to 300° C. by current supply is        repeated 30,000 times. In consideration of Young's modulus (73        GPa), thermal conductivity (420 W/m·K), electrical conductivity        (volume resistivity 1.6 μΩ·cm), cost required for acquisition,        and the like, it is most preferable to use the thermal stress        relieving layer made of Ag. Furthermore, from the viewpoint of        more reliably suppressing diffusion of Sn into the thermal        stress relieving layer, each of the first barrier layer and the        second barrier layer is preferably made of Ni and/or Ni alloy.        Moreover, from the viewpoint of increasing the thermal        conductivity of the entire bonding member, the total thickness        of the first Ag brazing material layer and the second Ag brazing        material layer is preferably 10% or less of the thickness of the        entire bonding member, and the first Ag brazing material layer        and the second Ag brazing material layer preferably contain 50        wt % or more of Ag. It is more preferable that the first Ag        brazing material layer and the second Ag brazing material layer        contain 60 wt % or more of Ag.

Hereinafter, a specific embodiment and examples of the semiconductordevice bonding member according to the present invention will bedescribed. A semiconductor device bonding member 10 of the presentembodiment is a member used for so-called die bonding, and is used forbonding a semiconductor device 20 to a substrate such as an electrodeand heat release substrate 30 as illustrated in FIG. 1 . Thesemiconductor device 20 is typically made of SiC, and the electrode andheat release substrate 30 is made of Cu. The semiconductor devicebonding member 10 of the present embodiment includes a thermal stressrelieving layer 11, a first Ag brazing material layer 12 formed on afront surface (a side of the semiconductor device 20), a second Agbrazing material layer 13 formed on a back surface (a side of theelectrode and heat release substrate 30), a first barrier layer 14provided between the stress relieving layer 10 and the first Ag brazingmaterial layer 12, and a second barrier layer 15 provided between thestress relieving layer 10 and the second Ag brazing material layer 13.The entire thickness of the semiconductor device bonding member 10 ofthe present embodiment is 10 μm or more and 300 μm or less. When thethickness is smaller than 10 μm, it is difficult to sufficientlyalleviate the thermal stress generated between the semiconductor device20 and the electrode and heat release substrate 30. On the other hand,when the thickness is larger than 300 μm, the thermal resistance of theentire semiconductor device bonding member increases, and the efficiencyof releasing heat from the semiconductor device decreases. Note that, inFIG. 1 , the size and thickness of each part are appropriately adjustedto facilitate understanding of an arrangement of each part.

Hereinafter, each part constituting the bonding member of the presentembodiment will be described.

(Semiconductor Device of Member to be Bonded)

In the semiconductor device 20 of a general IGBT, an electrode andbonding layer 21 made of Ni (for other special products, Co, Ag, Cu, ormultilayer may be used.) and also serving as an electrode and a bondinglayer is provided on a bonding surface of the semiconductor device. Inmany cases, the electrode and bonding layer 21 is only one layer made ofNi or Ni alloy, but in some cases, an antioxidant layer such as a thinAg layer or Au layer is further provided in order to prevent oxidationof Ni. Even when these antioxidant layers are provided, in the presentembodiment, the metal constituting the antioxidant layer diffuses intothe Ag brazing material layer 12 at the time of bonding and becomes apart of the Ag brazing material layer. Ni and Ag hardly form a solidsolution with each other, but when Sn is present, Ag and Ni react witheach other via Sn. Therefore, the Ag brazing material layer 13 maycontain an alloy containing other elements such as Ni, or Ag or Auconstituting the antioxidant layer, or an alloy of Ag or Au.

(Electrode and Heat Release Substrate of Member to be Bonded)

The electrode and heat release substrate 30 is basically made of Cu orCu alloy. When used at a high temperature, it is desirable to provide anantioxidant layer such as Ni or Ni alloy in order to prevent oxidation.In the present embodiment, a bonding surface of the electrode and heatrelease substrate 30 is subjected to electroless plating with Ni.

(First Ag Brazing Material Layer and Second Ag Brazing Material Layer)

The thermal stress relieving layer 11 made of pure Ag and the Ni layer21 provided on the bonding surface of the semiconductor device 20 can bebonded to the first Ag brazing material layer 12 under the conditionthat the semiconductor device 20 is not broken, and the condition thatthe first Ag brazing material layer passes the power cycle test at 300°C. is required to be satisfied. In the bonding member described inPatent Literature 1, since the semiconductor device is bonded at a lowtemperature, breakage of the semiconductor device can be avoided.However, fatigue deterioration caused by voids provided for relaxingthermal stress has occurred. In the semiconductor device bonding member10 of the present embodiment, the thermal stress relieving layer 11 isprovided separately from the first Ag brazing material layer 12 and thesecond Ag brazing material layer 13, and since the thermal stress isrelaxed by the thermal stress relieving layer 11, it is not necessary toprovide voids inside the first Ag brazing material layer 12 and thesecond Ag brazing material layer 13. Furthermore, these layers can bethinned. It is preferable that the thicknesses of the first Ag brazingmaterial layer 14 and the second Ag brazing material layer 15 areappropriately adjusted in a range of 10 μm or less in total. When thethickness of the first Ag brazing material layer and the second Agbrazing material layer are larger than 10 μm, voids and aggregation ofmetal compounds are likely to occur. Furthermore, in order to increasethe thermal conductivity of the entire semiconductor device bondingmember 10, the total thickness of the 1Ag brazing material layer 12 andthe second Ag brazing material layer 13 is desirably suppressed to 10%or less of the total thickness of the bonding member 10.

When the Ag brazing material layer is formed, an Ag foil and an Sn foileach having a thickness corresponding to a required composition ratioare used. The present inventors have found that by using foils of Ag andSn in a vacuum and increasing the temperature and load in the Ag brazingmaterial layer in order to require bonding reliability with a thinlayer, a gas generated when Sn is melted is easily released, and asufficiently thin layer with few voids can be produced, and further,bonding can be performed at 450° C. or lower at which a semiconductordevice is not broken, and for example, even a 65Ag35Sn alloy (Ag brazingmaterial) has a thermal conductivity of 110 W/m·K, and a thin Ag brazingmaterial layer with few defects and few voids of 5 vol % or less can beproduced.

The AgSn-based Ag brazing material has good wettability with Cu, Ag, Ni,and Au, and high bonding reliability can be obtained. At the time ofbonding, a metal constituting a layer provided on the surface of thesematerials for bondability and oxidation resistance may react with Ag orSn, but if the Ag brazing material is 50 wt % Ag or more, a sufficientlyhigh thermal conductivity can be secured even when such a reactionoccurs.

The Ag brazing material layer can be produced, for example, bysuperposing an Ag foil and an Sn foil, and heating and pressurizingthem. Alternatively, it can also be prepared by forming a very thin filmof Ag and Sn by sputtering, plating, or the like, or by providing athick foil, plate, or the like and heating and pressurizing the film onthe surfaces of the semiconductor device 20 and the electrode and heatrelease substrate 30, which are members to be bonded, and the membersconstituting the thermal stress relieving layer 11. Moreover, it canalso be prepared by providing a part or all of the Ag layer and the Snlayer on the member to be bonded and heating and pressurizing the Aglayer and the Sn layer. Note that the composition and thickness of thefirst Ag brazing material layer 12 and the second Ag brazing materiallayer 13 are not necessarily the same.

(Thermal Stress Relieving Layer)

In the bonding member 10 of the present embodiment, the thermal stressrelieving layer 11 made of Ag or the like is used in order to avoidbreakage (failure mode 2) occurring inside the bonding member 10. Thethermal stress relieving layer 11 is preferably a plate material or foilof pure Ag that is flexible (has a low Young's modulus) and has a highthermal conductivity. Alternatively, in a case where the large-sizedsemiconductor device 20 or the extra-large electrode and heat releasesubstrate 30 is bonded, a skeleton in which voids of 40 vol % or lessare formed in order to further reduce the Young's modulus may be used asthe thermal stress relieving layer 11. However, when the voids are 40vol % or more, the thermal stress relieving layer is buckled at the timeof bonding.

Since the Young's modulus of Ag is 73 GPa and is smaller than theYoung's modulus of SiC (410 GPa) and the Young's modulus of Cu (120GPa), the thermal stress generated between the semiconductor device 20and the electrode and heat release substrate 30 is relaxed bydeformation of the thermal stress relieving layer 11 made of Ag.Furthermore, since the thermal conductivity of Ag is as high as 420W/m·K, heat generated from the semiconductor device 20 can beefficiently released to the substrate. In the bonding member describedin Patent Literature 1, internal fracture occurs due to generation of anintermetallic compound with Sn in Ag. A plate material of pure Ag isformed by melting and rolling at a temperature higher than or equal tothe melting point of Ag of 960° C., and therefore even when the powercycle test at 300° C. is performed, no change due to fatigue degradationis observed at all. Furthermore, since a skeleton having a space insidea plate seat of pure Ag is also produced by heat sintering at about 900°C., no change due to fatigue degradation is observed even when the powercycle test at 300° C. is performed.

(First Barrier Layer, Second Barrier Layer)

When the first Ag brazing material layer 12 and the second Ag brazingmaterial layer 13 are provided directly on a surface layer of the pureAg plate material constituting the thermal stress relieving layer 11, Sncontained in these brazing material layers easily reacts with Ag in thethermal stress relieving layer 11, and the Sn easily diffuses into thethermal stress relieving layer 11. As a countermeasure in such a case, abarrier layer made of Ni may be disposed in the field of Ag brazing, andthis is also effective in the present embodiment. By disposing the firstbarrier layer 14 and the second barrier layer 15 on the front and backsurfaces of the thermal stress relieving layer 11 in this manner, onlypure Ag having low electric resistance is present inside the thermalstress relieving layer 11, and aggregation of an intermetallic compoundof Ag and Sn can be prevented to exhibit an original thermal stressrelieving effect of Ag. Furthermore, the thermal conductivity inherentto Ag does not decrease or the electrical resistance does not increase.Moreover, the aggregation of the intermetallic compound of Ag and Sn canbe prevented. As the thermal stress relieving layer 11, an appropriatematerial such as a Cu plate material, an Al plate material, or askeleton provided with a space inside the Cu plate material or the Alplate material can be used as long as required performance andcharacteristics are satisfied.

(Bonding)

In the present embodiment, the method is not particularly limited aslong as the semiconductor device 20 and the electrode and heat releasesubstrate 30 can be bonded at a low temperature of 450° C. or less inorder to prevent breakage of the semiconductor device 20. As one of theoptimal methods, for example, there is a method in which film-like orfoil-like melted Sn is diffusion-reacted with film-like or foil-like Agto produce an Ag brazing material. In the SiC semiconductor device, itis considered that the semiconductor device is not broken when thetemperature is about 450° C. and the pressure is about 30 MPa, but inorder to more reliably avoid breakage of the semiconductor device, it ispreferable to bond the semiconductor device at a temperature of 400° C.or less and a pressure of 15 MPa or less. Furthermore, the bonding timeis preferably not more than 60 minutes. Moreover, considering bonding ofa semiconductor device made of Si or GaN, it is more preferable tosatisfy the requirements of a temperature of 350° C. or less, a pressureof 5 MPa or less, and a bonding time of 10 minutes or less. The presentinventors have improved the Ag brazing material to enable bondingsatisfying these requirements. In the present embodiment, in order tosolve the problems of the failure modes 1 and 2, the first Ag brazingmaterial layer 12 and the second Ag brazing material layer 13 are bondedto the thermal stress relieving layer 11 at a low temperature asdescribed above. The bonding in the present embodiment is performed inconsideration of a bonding technique using solder or a sintered body ofAg nano-particles, and equipment and a production technique used forthese can be appropriately applied.

Next, a conventional technique of bonding a semiconductor device(conventional examples), examples corresponding to the above embodiment,and comparative examples in which a part of the configuration of theexamples is changed will be described.

In Conventional Examples 1 to 8, bonding of a SiC semiconductor deviceand a Cu electrode and heat release substrate was attempted usingbonding members shown in Table 1. Bonding conditions in each example arealso shown.

TABLE 1 Characteristics evaluation Bonding member Bonding conditionsThermal Overall Tem- Bond- conduc- thick- Void per- ing tivity nessfraction Bonding Atmos- ature Load time Bond- Power (W/m · (μm) Material(%) method phere (° C.) (MPa) (min) ability Cycle K) Conven- NoneHeating and Ar 350 5 10 x — — tional pressure bonding Example method 1Conven-  2 Ag layer (bonding — Heating and N2—H2 350 5 10 x — — tionalsurface of bonded pressure bonding Example member) method 2 Conven- 10060Ag30Cu10Sn — Heating and N2—H2 350 5 10 x — — tional pressure bondingExample method 3 Conven- 100 95Pb5Sn solder 5% or Heating and N2—H2 3501 1 ○ x — tional less pressure bonding Example method 4 Conven- 100Nano-Ag Un- Heating and Ar 350 5 10 ○ x — tional known pressure bondingExample method 5 Conven- 100 80Ag20Sn 40 Powder Sn Ar 350 5 10 ○ x —tional melting reaction Example bonding method 6 Conven-  10 80Ag20Sn 35Powder Sn Ar 350 5 10 ○ x — tional melting reaction Example bondingmethod 7 Conven- 100 90Ag10Sn  7 Powder Sn Ar 400 15 10 ○ x — tionalmelting reaction Example bonding method 8

In Conventional Example 1, a SiC semiconductor device and a Cu electrodeand heat release substrate were stacked, directly placed in an argonatmosphere without a bonding member, and directly bonded by heating to350° C. and pressurizing to 5 MPa for 10 minutes.

In Conventional Example 2, a 1 μm-thick Ag layer (2 μm-thick Ag layer intotal) was formed on each of a bonding surface of a SiC semiconductordevice and a bonding surface of a Cu electrode and heat releasesubstrate, and placed in a nitrogen and hydrogen atmosphere, and bothwere bonded by heating to 350° C. and pressurizing to 5 MPa for 10minutes.

In Conventional Example 3, an Ag brazing material (60Ag30Cu10Sn) havinga thickness of 100 μm was sandwiched between a SiC semiconductor deviceand a Cu electrode and heat release substrate, placed in a nitrogen andhydrogen atmosphere, heated to 350° C. and pressurized to 5 MPa for 10minutes, and bonded.

In Conventional Example 4, a SiC semiconductor device and a Cu electrodeand heat release substrate were bonded to each other with a Pb-basedsolder (95Pb5Sn solder) by heating the SiC semiconductor device and theCu electrode and heat release substrate to 350° C. and pressurizing theSiC semiconductor device and the Cu electrode and heat release substrateto 1 MPa for 1 minute in a nitrogen and hydrogen atmosphere. The voidfraction of the bonding member of Conventional Example 4 is 5 vol % orless.

In Conventional Example 5, a Ag nano-particle layer having a thicknessof 100 μm is provided between bonding surfaces of a SiC semiconductordevice and an electrode and heat release substrate, and the SiCsemiconductor device and the electrode and heat release substrate arebonded to each other by being placed in an argon atmosphere, heated to350° C., and pressurized to 5 MPa for 10 minutes.

The thickness of each of the bonding members of Conventional Examples 3to 5 is 100 μm.

Conventional Examples 6 to 8 correspond to the semiconductor devicebonding member proposed by the present inventors in Patent Literature 1.

In Conventional Examples 6 and 7, 80 wt % of Ag powder and 20 wt % of Snpowder were disposed between a SiC semiconductor device and an electrodeand heat release substrate, the SiC semiconductor device and theelectrode and heat release substrate were placed in a non-oxidizingatmosphere (argon), and Sn was melted and bonded by heating to 350° C.and pressurizing to 5 MPa for 10 minutes.

In Conventional Example 8, 90 wt % of Ag powder and 10 wt % of Sn powderwere disposed between a SiC semiconductor device and an electrode andheat release substrate, the SiC semiconductor device and the electrodeand heat release substrate were placed in an argon atmosphere, and Sn ismelted and bonded by heating to 400° C. and pressurizing to 15 MPa for10 minutes.

The bonding member of Conventional Example 6 contains 40 vol % of voids,the bonding member of Conventional Example 7 contains 35 vol % of voids,and the bonding member of Conventional Example 8 contains 7 vol % ofvoids. The thickness of the bonding members of Conventional Examples 6and 8 is 100 μm, and the thickness of the bonding member of ConventionalExample 7 is 10 μm.

For each of the above-described conventional examples, first, the stateof the bonded part was visually confirmed, and the bondability wasevaluated. In the evaluation of the bondability, a sample in whichcracking, chipping, or peeling did not occur in the bonding member orthe bonding part was determined to be acceptable. In ConventionalExamples 1 to 3, the semiconductor device and the electrode and heatrelease substrate were not bonded, and only Conventional Examples 4 to 8passed the evaluation of bondability.

(Power Heat Cycle Test)

A power cycle test was performed on Conventional Examples 4 to 8 thatpassed the evaluation of bondability. In the power cycle test, a heatingand cooling cycle was repeated 30,000 times in which a test pieceobtained by bonding a SiC semiconductor device and a Cu electrode andheat release substrate to each other was attached to a large-sizedwater-cooled cooler using thermal grease, the semiconductor device 20and the electrode and heat release substrate 30 were supplied currentfor 3 seconds to be heated to 300° C., and then the semiconductor deviceand the electrode and heat release substrate were cooled to 25° C. for30 seconds by the cooler. When an abnormal voltage or electric currentoccurred during the test, continuation of the test was dangerous, andthe test was stopped. When a crack or breakage occurred in a crosssection of the test piece, the test piece was determined to beunacceptable. Basically, the thermal conductivity after the power cycletest of 30,000 times was measured, and those having a thermalconductivity of 200 W/m·K or less were determined to be unacceptable.This makes it possible to capture a change that does not appear in thepower cycle test (for example, Non Patent Literatures 4 to 6). All ofConventional Examples 4 to 8 failed in the power cycle test. Note that,as in Examples described later, the following thermal conductivitymeasurement was performed for a test piece that passed the power cycletest.

(Measurement of Thermal Conductivity)

For the test piece that passed the power heat cycle test, the thermalconductivity was measured by a laser flash method using FTC-RTmanufactured by Advanced Riko Co., Ltd. In the laser flash method, thesurface of the sample is irradiated with pulsed laser light andinstantaneously heated, and a process in which the heat of the surfaceis diffused to the back surface of the sample with the lapse of time isobserved as a time change in the temperature of the back surface of thesample. The standard sample in this measurement method is a circularpiece having a diameter of 10 mm and a thickness of 1 mm, or a squarepiece having 10 mm square shape and 1 mm in thickness. This test piecewas obtained by bonding a 10 mm square, 1 mm thick Cu electrode and heatrelease substrate and a 5 mm square, 0.35 mm thick SiC semiconductordevice with a 5 mm square, 11 to 100 μm thick bonding member. In orderto correct the difference from the dimension of the standard sample inthe laser flash method, the thermal conductivity of the bonding memberwas calculated based on the measurement results of the Cu electrode andheat release substrate and the SiC semiconductor device having the sameshape as the test piece and the measurement results of the test piecesof the Examples described later.

As can be seen from the results shown in Table 1, when the SiCsemiconductor device and the Cu electrode and heat release substratewere bonded by the conventional technique, there is no piece whichpasses all the evaluations (tests). SiC semiconductor devices operatingat 300° C. are required to have a sufficient thermal conductivity evenafter the power cycle test in addition to the above test, but all thetest pieces failed before the evaluation.

Next, a test piece (hereinafter, it is simply referred to as “Example 1”or the like.) in which a semiconductor device and an electrode and heatrelease substrate are bonded to each other by a bonding member of anexample according to the present embodiment and a comparative exampleprepared based on the above results will be described. Table 2 shows theconfiguration of the bonding member and bonding conditions in eachexample. The thickness of the brazing material layer in Table 2 is thesum of the thicknesses of the first Ag brazing material layer 12 and thesecond Ag brazing material layer 13. In Examples 1 to 11, the same Agbrazing material layer was used as the first Ag brazing material layer12 and the second Ag brazing material layer 13, but the first Ag brazingmaterial layer 12. However, the second Ag brazing material layer 13 maydiffer from each other.

TABLE 2 Bonding member Bonding layer Barrier with SiC/Cu layerCharacteristics (Ag brazing (Ni, 1 evaluation Over- material layer)Thermal stress μm for Bonding conditions Thermal all Material Totalrelieving layer each of Tem- conduc- thick- (Main thick- Void Thick-Void upper per- tivity ness com- ness fraction ness fraction and BondingAtmos- ature Load Time Bond- Power (W/m · (μm) ponent) (μm) (%) Material(μm) (%) lower) method phere (° C.) (MPa) (min) ability Cycle K) Example100 80Ag20Sn 10 5% or Ag 88 0 Presence Low- Vac- 350 5 10 ○ ○ 329 1 lessplate temp- uum Example 100 80Ag20Sn 5 5% or Ag 93 0 Presence eratureVac- 350 5 10 ○ ○ 340 2 less plate Ag uum Example 100 80Ag20Sn 5 5% orAg 93 20 Presence brazing Vac- 350 5 10 ○ ○ 272 3 less plate ma- uumExample 100 80Ag20Sn 5 5% or Ag 93 40 Presence terial Vac- 350 5 10 ○ ○209 4 less plate bond- uum Example 300 80Ag20Sn 5 5% or Ag 293 0Presence ing Vac- 350 5 10 ○ ○ 357 5 less plate method uum Example  1180Ag20Sn 1 5% or Ag 8 20 Presence Vac- 350 5 10 ○ ○ 266 6 less plate uumExample 100 80Ag20Sn 0.5 5% or Ag 97.5 10 Presence Vac- 350 5 10 ○ ○ 3547 less plate uum Example 100 90Ag5Sn 5 5% or Ag 93 0 Presence Vac- 350 510 ○ ○ 333 8 less plate uum Example 100 90Ag5Sn 5 5% or Ag 93 0 PresenceVac- 400 10 10 ○ ○ 351 9 less plate uum Example 100 70Ag30Sn 1 5% or Ag97 0 Presence Vac- 350 5 10 ○ ○ 265 10 less plate uum Example 10060Ag40Sn 1 5% or Ag 97 0 Presence Vac- 350 5 10 ○ ○ 240 11 less plateuum Example 100 80Ag20Sn 5 5% or Cu 93 30 Presence Vac- 350 5 10 ○ ○ 22312 less plate uum Example 100 80Ag20Sn 1 5% or Al 97 0 Presence Vac- 3501 10 ○ ○ 208 13 less plate uum Compar- 100 70Ag30Sn 2 5% or Ag 98 0Absence Vac- 350 5 10 ○ x — ative less plate uum Example 1 Compar- 10080Ag20Sn 10 5% or Ag 90 20 Absence Vac- 350 5 10 ○ x — ative less plateuum Example 2 Compar- 100 80Ag20Sn 10 5% or Ag 88 50 Presence Vac- 350 510 x — — ative less plate uum Example 3 Compar- 100 80Ag20Sn 15 5% or Ag83 20 Presence Vac- 350 5 10 ○ x — ative less plate uum Example 4Compar- 100 80Ag20Sn 15 5% or Ag 83 20 Presence Vac- 400 10 10 x — —ative less plate uum Example 5 Compar- 100 80Ag20Sn 5 5% or Nano- 93 Un-Presence Vac- 350 5 10 ○ x — ative less Ag known uum Example 6

In each of Examples 1 to 13, a SiC semiconductor device and an electrodeand heat release substrate were bonded to each other by a semiconductordevice bonding member prepared by the above-described bonding method(low-temperature Ag brazing material bonding method in which Sn foil orlayer is melted and diffusion-reacted in Ag foil or Ag layer) underconditions of a temperature of 350° C. (the heating temperature was setto 350° C. higher than 300° C. which is a heating temperature in thepower heat cycle test.), a pressure of 5 MPa (10 MPa in Example 9), anda retention time of 10 minutes in a vacuum atmosphere.

Preparation and evaluation steps of Example 1, which is an example ofthe present embodiment, are as follows. The preparation and evaluationsteps are the same for Example 2 to 13 (the thickness and the like ofeach member are different individually).

Preparation Step 1; A (commercially available) SiC semiconductor device(5 mm square, 0.35 mm thick) in which a Ni layer having a thickness of 1μm is formed on a bonding surface is prepared.

Preparation Step 2; A (commercially available) electroless Ni plated 1μm Cu plate material (10 mm square, 1 mm thick) in which a Ni layerhaving a thickness of 1 μm is formed on a bonding surface is prepared.

Preparation Step 3; A layered product by forming Ni layers (barrierlayers) each having a thickness of 1 μm on upper and lower surfaces ofan Ag plate material having a thickness of 88 μm, and providing Ag foilsand Sn foils each having a thickness corresponding to the contents of Agand Sn in each Example (in Example 1, 80 wt % of Ag and 20 wt % of Sn)on the surface of each Ni layer.

Preparation Step 4; The SiC semiconductor device, the laminate, and theCu plate material are stacked and set in a hot press machine.

Preparation Step 5; In the hot press machine, the sample is held at atemperature of 350° C. and a pressure of 5 MPa (10 MPa only in Example9) for 10 minutes in a vacuum atmosphere.

Evaluation Step 1; Two test pieces are produced by the preparation steps1 to 5, and the bonding state is checked to evaluate the bondability.Then, 1 sample was heated to 500° C. and held for 30 minutes, and it wasconfirmed whether there was a problem in heat resistance. If there is noproblem in heat resistance, the power heat cycle test at 300° C. isperformed using the remaining one.

Evaluation Step 2; The thermal conductivity of the test piece thatpassed the power heat cycle test at 300° C. is measured by the laserflash method.

All of Examples 1 to 13 described above passed the bondability, heatresistance, and power cycle test. Moreover, as a result of measuring thethermal conductivity by the laser flash method performed after the powercycle test, the thermal conductivity was 200 W/m·K or more in all ofExamples 1 to 11.

On the other hand, among Comparative Examples 1 to 4 prepared bychanging any parameter from Examples 1 to 11, Comparative Example 3 hadpoor bondability, and all of other Comparative Examples 1, 2, and 4failed in the power cycle test.

Each of the above embodiment and examples is an example, and may beappropriately changed in accordance with the gist of the presentinvention. For example, although the above-described embodiment andexamples have a configuration including a single thermal stressrelieving layer, a structure in which a plurality of thermal stressrelieving layers are laminated with an Ag brazing material layerinterposed therebetween can also be adopted. In this case, the pluralityof thermal stress relieving layers are not limited to the same material,and some or all of the thermal stress relieving layers may be made ofdifferent materials. Furthermore, in the above embodiment, the thicknessof each of the first barrier layer 14 and the second barrier layer 15 is1 μm, but the barrier layer may be interposed, and the thickness may bereduced to about 0.5 km.

The semiconductor device bonding member according to the presentinvention can be suitably used, for example, for bonding a semiconductordevice and an electrode and heat release substrate (current supplysubstrate) in a power semiconductor module of a type in which a currentsupply path is formed in a direction perpendicular to a bonding surfaceof the semiconductor device. Of course, the present invention can alsobe suitably used in various fields (communication, calculation, memory,laser, LED, sensor, etc.) in which a semiconductor module of a type inwhich a current supply path is formed in a direction parallel to abonding surface of a semiconductor device is used. Furthermore, in theIGBT module, the present invention can also be used in a semiconductormodule on which a semiconductor device such as Si, GaN, or GaAs ismounted, other than a SiC semiconductor device. The semiconductor devicebonding member according to the present invention can greatly contributeto downsizing, high performance, and cost reduction of futuresemiconductor modules. Moreover, in the present specification, thesemiconductor module has been mainly described, but the semiconductordevice bonding member according to the present invention can besimilarly used for a semiconductor package. For example, thesemiconductor device bonding member according to the present inventioncan also be suitably used for bonding a member having a large differencein linear expansion coefficient, such as a ceramic substrate, to anelectrode and heat release substrate made of Cu or the like.

REFERENCE SIGNS LIST

-   -   10 . . . Semiconductor Device Bonding Member    -   11 . . . Thermal Stress Relieving Layer    -   12 . . . First Ag Brazing Material Layer    -   13 . . . Second Ag Brazing Material Layer    -   14 . . . First Barrier Layer    -   15 . . . Second Barrier Layer    -   20 . . . Semiconductor Device    -   21 . . . Layer which shares Bonding Layer Metal with Electrode        of Semiconductor Device    -   30 . . . Electrode and Heat Release Substrate

1. A semiconductor device bonding member used for bonding asemiconductor device and a substrate, the bonding member comprising: athermal stress relieving layer made of any of Ag, Cu, Au, and Al; afirst Ag brazing material layer containing Ag and Sn as main componentsand provided on a side of the thermal stress relieving layer to whichthe semiconductor device is bonded; a second Ag brazing material layercontaining Ag and Sn as main components and provided on a side of thethermal stress relieving layer to which the substrate is bonded; a firstbarrier layer made of Ni and/or Ni alloy and provided between the stressrelieving layer and the first Ag brazing material layer; and a secondbarrier layer made of Ni and/or Ni alloy and provided between the stressrelieving layer and the second Ag brazing material layer, wherein athermal conductivity is 200 W/m·K or more after a power cycle test inwhich heating to 300° C. by current supply and cooling to 25° C. arerepeated 30,000 times.
 2. The semiconductor device bonding memberaccording to claim 1, wherein the thermal stress relieving layer is madeof Ag.
 3. The semiconductor device bonding member according to claim 1or 2, wherein a porosity of the first Ag brazing material layer and thesecond Ag brazing material layer is 5 vol % or less, and a totalthickness of the first Ag brazing material layer and the second Agbrazing material layer is 10% or less of a thickness of the entirebonding member.
 4. The semiconductor device bonding member according toany one of claims 1 to 4, wherein the thermal stress relieving layer hasa porosity of 40 vol % or less.
 5. The semiconductor device bondingmember according to any one of claims 1 to 5, wherein an overallthickness is 10 μm or more and 300 μm or less.
 6. A semiconductor modulecomprising the semiconductor device bonding member according to any oneof claims 1 to 6.